Hot roll fusers are frequently used to fix toner images in the so-called electrophotographic process. In such processes, toner is transferred from a photo-sensitive drum to paper, then heated (for example, using a hot roll fuser), thereby melting thermoplastic components of the toner and fixing the image to the paper. The electrophotographic process has found wide use in, for example, dry copying machines, laser printers, LED printers, and printing units of facsimile systems. To permit the use of a compact lightweight fixing unit in the electrophotographic process and to allow the unit to be heated to its operating temperature in a shortened period of time, the fixing element is frequently in the form of a cylindrical aluminum tube, which acts as a roller, having a halogen lamp inserted therein to heat the roller.
When the printing device is switched on, the roll goes through an initial heating period, called the transitory state, during which time the roller is heated up to operating temperature of from about 180.degree. to 200.degree. Celsius. At that point, the roll is in a steady state temperature mode during which time the temperature is maintained at operating (i.e., fixing) temperature. Maintaining a constant hot roll temperature along the length of the roll, as well as under different operating conditions from transient to steady state, is difficult, especially at higher printing speeds. Current hot fuser rolls contain a heating lamp with an energy profile that is boosted at its ends relative to its center by about 50% to try to compensate for roller heat loss which occurs at the end of the rolls. During the transient state or initial heating of the roll, the temperature at the center of the hot roll is greater than the temperature at the ends. As pages are printed, the center and end temperatures of the roll cross over. During steady state operation of continuous printing at higher printing speeds (e.g., about 34 pages per minute (ppm)), the temperature at the center of the hot fuser roll has been measured to be as much as 23.degree. C. less than the temperature near the ends of the roll. To overcome the steady state drop in temperature at the center of the roll, a heating lamp profile with less boost at its ends is desirable. However, as the lamp profile becomes more uniform, the hot roll temperature at the center of the roll tends to overshoot its operating temperature more during the transient state. Thus, the ability to maintain a relatively uniform temperature across the length of the fuser roll over both the transient and steady state processes is very difficult to achieve. Yet, it is very important to achieve hot roll temperature uniformity since major differences in temperature across the roll result in different qualities of print across the printed page, as well as to wrinkling and other paper damage in various areas of the printed page. Very complex solutions have been proposed for dealing with this issue. For example, some current printers contain complex control algorithms that gradually increase the target or set point temperature at the thermistor position of the hot fuser roll in order to limit the amount of overshoot at the roll center.
A typical fuser roll used in current products, shown in FIG. 2, has a uniform wall thickness across its length. The heating lamp power profile is boosted fifty percent at its end portions, compared to the center portion. The numbers from (1) to (13) in FIG. 2 are used to indicate the positions at which the temperature of the roll is measured, for experimental purposes.
The temperature response and temperature profile for this typical roll are shown in FIGS. 3, 4 and 5. There is a greater than 35.degree. C. temperature droop from center to end in the transient state, and a 23.degree. C. temperature droop from end to center in the steady state. These temperature droops will result in poor printing quality, especially for those products with higher printing speeds because of the shorter residence time of the paper at the fuser roll.
As print speeds increase and it becomes increasingly difficult to achieve fuse grade, it is very desirable to eliminate the variations in temperature described above. Any temperature gradient from end to center of the roll or from transient to steady state potentially reduces fuser performance. Since printer manufacturers are bound on the upper end of the temperature range by material properties and safety concerns, a wider operating temperature range results in a lower minimum operating temperature and thus poorer fuse grades with all else being equal. It would, therefore, be highly desirable to be able to achieve uniformity of temperature on the fuser roll both across the roll lengthwise from end to center and back to end, as well as between the transient and steady states, in a manner which is simple, effective and which does not require extensive modifications to the structure or control software of the printer. The present invention achieves this result in a very simple and effective manner by distributing, in a specifically-defined manner, the thermal mass across the length of the hot fuser roll.
U.S. Pat. No. 5,304,784, Tagashira et al., issued Apr. 19, 1994, describes a resistance heater used for fixing toner in an electrophotographic process. The purpose of the invention is to even out the heat through the fixing process. This is done by manipulating the amount of thermal insulation surrounding various portions of the heater, thereby controlling the amount of heat dissipation from the heater. The heater is designed such that it has a greater amount of insulation at its ends than in the center.
U.S. Pat. No. 5,724,639, Tamura et al., issued Mar. 3, 1998, describes a fixing roller for use in an electrophotographic process which includes an internal heating lamp located within the roller. Black, transparent, silver and white coatings are applied to various portions of the internal roller surface to even out the surface temperature of the roller during the fixing process.
U.S. Pat. No. 4,937,600, Hirabayashi, issued Jun. 26, 1990, describes an algorithm and techniques for adjusting the temperature on the surface of a fixing roll used in an electrophotographic process. This is accomplished primarily by using two different heating voltages which are blended, as appropriate, to keep the fixing temperature within the desired range. Fixing rolls disclosed are shown to have a uniform thickness across their length. The patent teaches, at column 12, that a thicker roll exhibits a slower temperature rise during initial heating. Further, the patent teaches, at columns 5-6, that an increase in roll heat capacity, which is achieved by pre-rotation of the fuser roll in contact with the pinch roller, results in less temperature overshoot during the transient heating phase.
U.S. Pat. No. 4,585,325, Euler, issued Apr. 29, 1986, describes a fixing roll for use in an electrophotographic process. The interior portion of the roll includes two end sections and one middle section, each of which includes separate heating coils. Temperature sensors measure the roll surface temperature and manipulate the heating coils relative to each other in order to make the temperature more uniform over the roll surface. No manipulation of the thermal characteristics of the roll itself is taught.
U.S. Pat. No. 4,639,990, Schiel, et al., issued Feb. 3, 1987, describes a calendar roll for use in a paper making, not an electrophotographic, process. The interior of the roll contains chambers which hold fluid. By controlling the temperature of the fluid in various chambers, the temperature of the roll at various points can be adjusted or made uniform across the length of the roll.
U.S. Pat. No. 5,331,384, Otsuka, issued Jul. 19, 1994, describes a heat fixing apparatus for an electrophotographic process. The Otsuka patent recognizes the problem of temperature differential across the length of the fixing roll and addresses this problem by putting a temperature detection device on the roll beyond the point where the paper passes (i.e., close to the end of the roll) and including an algorithm which adjusts heating of the roll based on that temperature and the size of the paper being printed. The fixing rolls disclosed are of uniform thickness and there is no suggestion to adjust the thickness of the fuser roll.
U.S. Pat. No. 5,481,346, Ohzeki, et al., issued Jan. 2, 1996, describes a heat fixing apparatus for use in an electrophotographic process. This patent recognizes the problem of temperature differentiation between the center and ends of the fuser roll and describes a hardware and software device for minimizing this differential. This is precisely the type of complex solution to the fuser roll temperature problem which the present invention addresses. The fixing rolls disclosed have a uniform thickness and there is no suggestion to adjust the thermal mass of sections of the roll. Similarly, see U.S. Pat. No. 5,742,865, Yajima, et al., issued Apr. 21, 1998.